We describe the evolution and microstructure of Si/CoSi2/Si (100) and (111) heterostructures formed by Co+ ion implantation into Si substrates (“mesotaxy”), followed by high temperature annealing. It is shown that the CoSi2 precipitate nucleation and ripening process, and eventual coalescence into buried layers, is controlled by interfacial structure and energetics. Understanding and control of these processes allows for the first time synthesis of otherwise almost identical CoSi2 buried layers with either twinned or untwinned CoSi2/Si(111) interfaces.

Tungsten and tungsten-silicides are of interest for semiconductor technology because of their refractory nature, low electrical-resistivity and high electromigration-resistance. This paper presents the first formation of buried tungsten-silicide layers in silicon, by proximity adhesion. The interlayers, created by a combination of chemical vapor-deposition (CVD) and proximity-adhesion were studied using transmission electron-microscopy (TEM). The behavior of the layers in the presence and absence of an adjacent silicon-dioxide interlayer was also investigated. Buried suicide layers were successfully formed with or without the adjacent silicon-dioxide. The suicide formed continuous layers with single grains encompassing the width of the interlayer. Individual grains were globular, with cusps at grain boundaries. This caused interlayer-thicknesses to be non-uniform, with lower thickness values being present at the cusps. Occasional voids were observed at grain-boundary cusps. The voids were smaller and less frequent in the presence of an adjacent oxide-layer, due to flow of the oxide during proximity adhesion. Electron-diffraction revealed a predominance of tungsten-disilicide in the interlayers, with some free tungsten being present. Stresses in the suicide layers caused occasional glide dislocations to propagate into the silicon substrate beneath the interlayers. The dislocations propagate only ∼100 nm into the substrate and therefore should not be detrimental to use of the buried layers. Occasional precipitates were observed at the end of glide-loops. These possibly arise due to excess tungsten from the interlayer diffusing down the glide dislocation to finally precipitate out as tungsten-silicide.

Vapor phase synthesis approaches were employed to produce laminated microstructures with high temperature phases alternated with ductile phase layers for toughness enhancement. Microlaminates of alternating layers of Mo and cosputtered Mo-Si were deposited on a Si substrate using direct current (DC) and radio frequency (RF) sputtering technique. Microstructural evolution and phase formation after diffusion annealing (900°C, 2 hr) was evaluated using cross-sectional transmission electron microscopy (TEM). Heat treating the microlaminates resulted in the growth of a Mo3Si interface phase between Mo and cosputtered layer and conversion of the amorphous cosputtered layer into crystalline Mo5Si3. A higher Si/Mo ratio in the cosputtered layer caused the formation of dispersed Mo5Si3 grains in Mo layer. The residual stress state was measured by monitoring the curvature change of the microlaminates using a laser beam. By decreasing the bilayer thickness to 1/3, the residual stress is reduced by ∼50%.

This study explores the relations between processing routes, microstructures and mechanical properties of the matrix/reinforcement interfaces in MoSi2/Nb composites. It was found that the fracture energy of the interfacial region depended on the interfacial bond strength, roughness of interface, and the nature of the interfacial compounds. The fracture energy between the oxide coating and intermetallic interfacial compounds was found to be lower than that between two intermetallics or between Nb and an intermetallic. Processing routes were found to affect the fracture energy of the interfacial region by changing interphase formation, changing microstructure of materials adjacent to the interface, or changing roughness of interface.

We have investigated the effect of an interfacial Ti layer on the mechanism of CoSi2 formation. The presence of this Ti intcrlayer shifts the completion of all the cobalt suicide reactions to higher temperatures, reduces the sensitivity of these reactions to the presence of the native oxide on silicon, and greatly modifies the morphology of the CoSi2/ < 100 > Si interface. The modification of the interface morphology arises from the formation of highly < 110 > oriented or < 100 > epitaxial CoSi2 and results in greater thermal stability of the disilicide.

Reactions in solids occur at interfaces. Following interfacial reactions has been difficult due to the small fraction of material making up the interfaces. In this paper, variable-temperature grazing x-ray diffraction is used to probe interfacial reactions in modulated ultra-thin film layered composites. This technique permits the monitoring of solid-state reactions in their earliest stages. The initial evolution of titanium-silicon interfaces within a superlattice is discussed. Our data suggest that the classical approach of modeling diffusion via Fick's laws is insufficient to describe the initial interdiffusion or mixing reaction at an interface.

In this work, Co/Ti multilayers were deposited on Si-(100) substrates by dual source thermal evaporation. Oxygen was found to be selectively incorporated into the Ti layers during the deposition. The samples were then treated by a two step annealing process. A —6nm Co2Si + CoSi2 layer was formed at the original Ti/Si interface after a 550°C, 2hr. annealing. The Ti(O) layers acted as selective diffusion membrane for Co and Si during this interaction. The morphology of the suicide layer was dependent on the thickness of the Ti(O) barrier layer. Selective removal via wet etching of the upper layers and a 750°C annealing produced stoichiometric and uniform epitaxial cobalt disilicide with a reduced film resistivity. A very flat top surface with no sign of groving or agglomeration and a much improved interface with the Si substrates were obtained.

The structure change of a Cr-Si-O thin film with regard to heat-treatment was investigated not only by the transmission XAFS method but by the surface sensitive XAFS method using synchrotron radiation. As a result of transmission XAFS, the Cr-Si-O thin film as sputtered has an amorphous structure like a mixture of SiO2, Cr and CrSix. After heat-treatment to 650 K, Si-Cr bonds decreased and Si-O and Cr-Cr bonds increased. CrSix is unstable in the system. The interfacial studies by the surface XAFS method showed i) at the interface with polyimide, there is a thin layer which is dominantly made of Cr2O3 and ii) at the interface with Al, Cr atoms are mainly coordinated to Cr. Analyses by the XAFS method gave consistent results with chemical analyses by x-ray photoelectron spei roscopy and observation by transmission electron microscopy.

Script lamellar microstructures which occur in certain intermetallic eutectic alloys, such as in the Mo-Si system, may provide desirable mechanical properties. Arc-melted specimens of MoSi2-Mo5Si3 eutectic alloys which exhibit the interlocking lamellar phases were examined in this study. We have observed, by conventional transmission electron microscopy (TEM), an orientation relationship between the MoSi2 and the Mo5Si3 phases, (110)[001]1-2 ‖ (330)[110]5-3, which is consistent with the crystallographic coordinate transformation matrix. High resolution transmission electron microscopy (HRTEM) discloses the interfacial dislocations to be of <100> and 1/2<111> types and the interfaces are regularly faceted. A shear fault which may be consisting of antiphase boundary (APB)-coupled 1/6<331> superpartial dislocations was observed in MoSi2 grain near the interface. Crack propagation paths suggest that the eutectic has a strong, low energy interface which is consistent with the observations of a low index orientation relationship between the two phases and the faceted interfacial structures.

Variable angle spectroscopie ellipsometry (VASE), a nondestructive optical technique, was used to characterize two different multilayer samples, each having a low-pressure chemical vapor deposited polycrystalline silicon (poly-Si) layer. Analysis of these samples by cross-sectional transmission electron microscopy (XTEM) revealed large changes in grain size, between the undoped, as-deposited, and doped, annealed poly-Si layers. Roughness at the top of the poly-Si layers was also observed by XTEM. These features, together with the other structure parameters (thickness and composition), were analyzed ellipsometrically by fitting the measured VASE spectra with appropriate multilayer models. Each composite layer (surface overlayer, interfacial layer, and poly-Si layer) was modeled as a physical mixture, using the Bruggeman effective medium approximation. The ellipsometrically determined thicknesses were in very good agreement with the corresponding results measured by XTEM. Furthermore, VASE analysis provided additional information about the relative fractions of the constituent materials in the different composite layers. Thus, it quantitatively characterized the surface and interracial properties, and also the doping and annealing effects on the microstructure of poly-Si layers.

The thermal oxidation of silicon in dry oxygen is examined with three different thickness measurements: Ellipsometry, transmission electron microscopy and step-profile measurements. The oxidation kinetics follow a linear-parabolic relationship throughout the measured thickness range. Previous deviations from linear-parabolic behavior result from inaccurate ellipsometer measurements of film thickness for films thinner than 0.05 /zm.

Samples of crystalline (111) silicon were coated with various thicknesses of hydrogenated amorphous silicon (a-Si:H), then coated with 100 nm of palladium. These samples were then reacted to form Pd2Si in vacuum. The activation energies and reaction prefactors were determined by monitoring the film thickness using x-ray diffraction and by 4-point resistivity measurements. The crystallographic texture of the metal overlayer and suicide films were investigated before and after the reaction.

Atom by atom deposition techniques, such as magnetron sputtering, make possible the synthesis of artificial superlattices with composition modulations in one dimension on an atomic scale[1]. Generaily, elastic effects (lattice mismatch) and chemical affinity (ordering or clustering tendencies are recognized as the most important driving forces for the thermodynamic stability of such materials. In this study the chemical aspect of stability will be examined using a formalism based on first-principles electronic structure calculations developed originally for bulk alloys. The formalism has been applied to a number of systems with fee and bec transition metals as constituents. In some cases the stability of artificial superlattices was found to depend in a rather surprising and unexpected way on the direction and wavelength of the composition modulation.

Interfaces are difficult to probe because the volume of the interfacial region is exceedingly small compared with the bulk. Our approach is sequentially to deposit thin (5–50Å) elemental layers to create a unique initial reactant. Due to the short modulation length, the interfaces constitute a large fraction of the total composite. Composites modulated below a critical repeat unit length are found to evolve into homogeneous amorphous alloys. Composites modulated above this critical distance behave like bulk diffusion couples. The layered nature of the starting reactant permits the evolution of the interfacial structure to be followed in a quantitative manner using x-ray diffraction. Distinct differences in the evolution of the interfaces above and below the critical distance are observed. This approach to studying interfaces also permits the direct and selective synthesis of crystalline compounds. Nucleation is the rate-limiting step in the formation of a crystalline material from an amorphous alloy, and the phase formed is strongly influenced by the stoichiometry of the homogeneous amorphous alloy. Results from studies of molybdenum-selenium and iron-silicon ultrathin-film composites will be discussed.

The atomic structure of a series of low-dimensional Fe/Tb multilayered structures has been explored using a conversion-electron, extended x-ray absorption fine structure (EXAFS) technique. A structural transition from a close-packed amorphous structure to a body-centered crystalline structure is detected to occur over an Fe layer thickness range of 12.5 Å to 15.0 Å (Tb thickness is held constant at 4.5 Å). Magnetic properties, specifically, magnetization, anisotropy field, and Kerr rotation angle, are measured and found to change significantly in response to this transition. Exploitation of the polarization properties of synchrotron radiation allowed for the description of the atomic structure both perpendicular and parallel to the sample plane.

The structures of e-beam evaporated Pd/V multilayer thin films, which were fabricated at different substrate temperatures, have been characterized by high-angle annular dark-field microscopy and high resolution electron microscopy techniques. X-ray scattering and crosssectional electron microscopy showed that both the Pd and V layers are composed of small textured crystallites with dominant orientations of Pd (111) and V (110). It is found that Pd/V multilayers with high chemical modulation can be fabricated at substrate temperatures around 350 K and at a deposition rate of 0.2 nm/s. Here high-angle annular dark-field microscopy has been shown to provide direct information about the compositional variation of the interlayers of these ML.

DC magnetron sputtering technique is used to fabricate inconel/carbon multilayers (ML) for applications in soft x-ray optical systems. The ML films were characterized by small angle x-ray scattering (SAXS), high resolution electron microscopy (HREM) and high-angle annular darkfield (HAADF) microscopy techniques. The HREM showed that the ML films are composed of smooth layers of amorphous components. The HAADF showed strong interdiffusion between inconel and carbon. There is no indication of any pure inconel or carbon regions in the ML films.

X-Ray reflectivity enables the determination of interface and surface roughness along with the variations present in the electron density. Total reflection X-ray fluorescence allows surface analysis with high sensitivity and quantification. By use of grazing angle x-ray fluorescence taken simultaneously with the reflectivity measurements, over a range of angles near the critical angle, it is possible in principle to produce a depth profile of each element, with a composition sensitivity of 0.0002%. A silicon-germanium single layer was used to calibrate the instrument and a Si-Ge 5- period superlattice for a demonstration measurement.

Glancing angle EXAFS and x-ray reflectivity are used to study the interface reaction between nickel-chromium alloys and aluminum. The two metals are found to react independently with Al, with the first reactions taking place at temperatures similar to those found for the pure metals. This means that Ni reacts first with Al to form NiAl3, leaving behind a Cr-rich region at the interface. In this Cr-rich region some of the Cr transforms to the bcc structure from the fcc form of the alloy. At higher temperatures Cr reacts to form CrAl7, and there is no evidence for ternary compound formation. Samples were also prepared with a controlled O contamination at the interface, and it inhibits the reaction much the same as for the pure metal cases. The Ni reaction is not identical to the pure sample case, since the presence of Cr slows down the reaction, and inhibits the initial reaction in the as-prepared bilayers.

A predictable, controllable approach to the synthesis of ternary compounds through known intermediates is presented. Thin and ultrathin film superlattices were made in the Mo-Se, Cu-Se and Mo-Cu systems. Differential scanning calorimetry, low- and high-angle x-ray diffraction were used to assess the interdiffusion and nucleation reactions between elemental layers in these one-dimensional crystals. The experimental parameter modulation distance was used to influence the interfacial reactions. The results from each binary system were then used to predict the reaction pathway in the synthesis of a ternary compound, Cu2Mo6Se8. Superlattices with two different lengthscales were investigated. In the first, only one intermediate, MoSe2 which typically crystallizes at ∼200'C, is observed prior to the crystallization of Cu2Mo6Se8. In the second, no crystalline intermediates are observed below 6000 C.